The present invention relates generally to paint spray guns, and more particularly to a high volume, low pressure spray gun having a controlled air cap pressure through a range of spray patterns.
In recent years, there has been an ever increasing industry and legislative drive towards increasing the transfer efficiency of fluid materials sprayed from pressurized spray guns. Transfer efficiency can be defined as the amount of sprayed fluid material, such as paint, that goes onto subject parts as compared to the amount lost to over spray and bounce back. A high transfer efficiency decreases fluid material consumption, reduces undesirable deposits on adjacent surfaces, and results in relatively less over spray which improves operator visibility. Importantly, transfer efficiency is a measure of the amount of fluid material dispersed into the ambient air which contributes to environmental pollution.
One class of spray gun uses pressurized air for atomizing liquid material and for shaping the envelope or pattern of the atomized liquid material as it is discharged from a nozzle assembly on the gun. Air atomization spray guns broadly fall into two classes. One type of air atomization spray gun uses a low volume flow of high pressure air (LVHP) for atomization and pattern shaping. The air pressure in such guns may typically be in the 40 psi to 100 psi range. The transfer efficiency associated with such guns, however, are far from optimal. This is due to the relatively high air pressures which produce a high degree of over spray and bounce back.
The other broad type of spray gun which uses pressurized air for atomizing liquid material employs a high volume, low pressure (HVLP) spray approach in order to increase fluid material transfer efficiency. The transfer efficiency of HVLP spray guns is much greater than the LVHP spray guns. HVLP atomization utilizes a high volume of air typically delivered at 10 psi or less to atomize fluid material. It is the large volume of air passing in contact with a fluid material in a suitable nozzle assembly which causes atomization of the fluid material.
Many industries have adopted the HVLP approach, either voluntarily or by legislative mandate. For example, currently the Southern California Air Quality Management District's rules and the EPA's National Emission Standards for Hazardous Air Pollutants require spray gun air cap pressure to be no greater than 10 psi.
One characteristic of HVLP spray guns is the variation in air cap pressure from full open spray pattern to closed spray pattern. Painters would prefer a consistent air cap pressure while adjusting the spray or fan pattern to stay in regulatory compliance so as to not exceed a 10 psi requirement, for example. By their design some spray guns vary 4 to 5 psi from full open spray pattern to fully closed. For example, if the spray gun is adjusted to provide an allowable 10 psi at a full open spray pattern and during the application the painter decides to close down the spray pattern, the air cap pressure may be increased to approximately 14 to 15 psi. Thus, the process would become out of regulatory compliance. If the painter attempts to compensate by adjusting the spray gun such that a full open spray pattern is achieved at 5 psi, then closing down the spray pattern during an application may result in an insufficient air cap pressure to atomize the paint for a quality finish.
It is therefore evident that there exists a need in the art for a high volume, low pressure spray gun which facilitates operation through a range to spray pattern adjustments while mitigating significant variations of the air cap pressure in comparison to the prior art devices.